Color Space

Considerations for Document Imaging

Color images can be described with three components, commonly Red, Green, and Blue. Therefore any color could be described in a three dimensional graph, called a color space. Mathematically the axis can be tilted or moved in different directions to change the way the space is described, without changing the actual colors. The values along an axis can be linear or non-linear. This gives a variety of ways to describe colors, that have an impact on the way we process a color image.

RGB

Familiar "Red Green Blue" or "RGB" is used in cameras, monitors and scanners. Combine (add) the three components to make white. "Nothing" (zero values for all the components) create black. These colors are called additive colors since they add together the way light adds to make colors, and is a natural color space to use with video displays.

RGB is not used in most printers, because printers deal with light that is reflected from the paper, rather light that shines on the paper.

Mathematically there is a simple relation: for example, white is approximately 30% red, 60% Green, 10% Blue. This is not a precisely defined and calibrated system.

Grey is any value where R=G=B, thus it requires all three (RGB) signals to produce a "black and white" picture. In other words, a "black and white" picture must be computed - it is not inherently available as one of the components specified.

Compression in the RGB color space is generally not as good as other color spaces. For example, note that blue is only a small factor in the gray/white, and the eye is less sensitive to blue. Therefore less precision is required for the blue signal, but it is equal to the others.

CYM, CYMK

Cyan, Yellow, Magenta. Similar to RGB, but from the opposite perspective, used in printing and painting. The three colors together absorb all the light that strikes it, appearing black (as contrasted to RGB where the three colors together made white). "Nothing" on the paper is white (as contrasted to RGB where nothing was black). These are called the subtractive or "paint" colors.

In practice, it is difficult to have the exact mix of the three colors to perfectly absorb all light and thus produce a black color. Expensive inks are required to produce the exact color, and the paper must absorb each color in exactly the same way. To avoid these problems, a forth color is often added - blacK - creating the CYMK color "space", even though the black is mathematically not required.

YUV, YIQ

The luminance (brightness), "Y", is retained separately from the chrominance (color). This is a simple mathematical transformation from RGB: Y is approximately 30% Red, 60% Green, and 10% Blue, the same as the definition of white above.

The YIQ color space is a simple variant of the YUV space, with the I and Q representing different colors than the U and V, to make better use of the precision of the numbers
I = .74 V - .27 U
Q = .48 V + .41 U

The YUV color space is commonly used in European television. The YIQ color space is commonly used in North American television systems. Note that in either case, if the chrominance is ignored, the result is a "black and white" picture.

YCbCr

During development and testing of JPEG it became apparent that chrominance sub sampling in this space allowed a much better compression than simply compressing RGB or CYM. Subsampling means that only one half or one quarter as much detail is retained for the color as for the brightness.

CIELUV, CIELAB

The previous color spaces were mathematically linear. However, the eye's response to color is non-linear. A certain change in a color component could cause a large or small change in the perception of the color. CIELAB, CIELUV (additive and sub-tractive color spaces) have uniform perception, but are mathematically non-linear, and therefore are much more complex.

The CIE... color spaces are formally defined and calibrated, so reproducible (exact color matched) colors are possible in both displays and printers.

The color space choice

In August 1991, the international Group 4 color fax committee decided to assume YCbCr would be the standard as they continued their studies. They noted that YCbCr was mandatory for compatibility with business image systems such as desktop publishing. For professional graphics, it was mandatory along with CIELAB for calibration. At the high end of publishing, many color spaces had to be supported, including YCbCr. In fact, YCbCr was the most widely used color space in all areas.

By the November 1992 Group 4 color fax meeting in Tokyo, CIELAB 1976 was selected as the primary color space, with YCbCr as one of several secondary options. Some of the people involved argue that the particular meeting was dominated by people with special interests, and don't believe that decision will stand.

If CIELAB becomes the fax standard, it would logically be our choice. However, YCbCr is much more widely used, and preferred by many technical experts.

Recommendation for color pictures: Use YCbCr, with 2:1 horizontal chrominance sub sampling (some-times called YYCbCr space) and optionally support other color spaces.